1. Technical Field of the Invention
The present invention relates generally to a contact slidable structure formed by a resilient terminal and a terminal electrode of a ceramic element such as a gas sensing element used in a gas sensor capable of measuring concentration of a predetermined component contained in an automotive exhaust gas.
2. Background Art
In an exhaust gas system of an automotive internal combustion engine, a gas sensor for measuring concentration of a predetermined component, such as oxygen gas, NOx and others, is installed to control combustion in the engine and to detect deterioration in a catalyst installed in the automotive exhaust gas system.
Japanese Patent Application Laid-open No.2002-286681 and U.S. Pat. No. 6,447,887 (Japanese Patent Application Laid-open No.2001-188060) disclose conventional gas sensing element.
As a type of the gas sensing element being installed in the gas sensor, a build-up gas sensing element having the structure as shown in FIGS. 5 and 6 mentioned later is known.
As shown in FIG. 6 mentioned later, the above gas sensing element has a ceramic body, an inner lead built in the ceramic body, a terminal electrode formed on an upper surface of the ceramic body, and a conductive through hole extending between the inner lead and the terminal electrode to form an electrical connection between the inner lead and the terminal electrode.
Furthermore, in order to feed electric power to the gas sensing element or pick up an output from the element, a resilient terminal made of a resilient metal is used for being in contact with the terminal electrode of the gas sensing element, as shown in FIG. 1 referred to later and in FIGS. 3,11,12 and 16 of U.S. Pat. No. 6,447,887.
However, heretofore, the following problems are known to arise in case of using the resilient terminal having the above structure.
More specifically, strength of the terminal electrode is deteriorated at the portion thereof where the conductive through hole is formed. Thus, cracks running from an edge of the thorough hole to the ceramic body may be generated, due to restoring force of the pre-stressed resilient terminal when the resilient terminal slides over the conductive through hole.
As shown in FIG. 16, the resilient terminal 99 slides leftwards in FIG. 16 while being in contact with the ceramic element 9 having the ceramic body 90, the inner lead 91, the terminal electrode 92 and the though hole 93 formed between the inner lead 91 and the terminal electrode 92, to form a contact slidable structure between the inner lead 91 and the terminal electrode 92. The restoring force of the resilient terminal 99 applied to the ceramic element 9 when the resilient terminal 99 slides over the ceramic element 9 while being in contact with the ceramic element 9 causes the crack 901 running from an edge of the conductive through hole 93 to the ceramic body 90.
Furthermore, as shown in FIG. 17, the insulating layer 95 may be formed onto the back surface of the inner lead 91 of the ceramic element 9. In case that the opening 930 penetrating into the conductive through hole 93 is formed by a pressure applied from the resilient terminal sliding over the conductive through hole 93, the ingredients contained in the insulating layer 95 formed under the conductive through hole 93 may exude through the opening 930 formed in the conductive through hole 93 over the upper surface of the terminal electrode 92 to form the insulating coating 931, which may cause conductive deterioration between the resilient terminal 99 and the terminal electrode 92. The present invention is produced in view of the above problems of the background arts.